1. Field of the Invention
This invention relates to a die attach material that has significant shear strength and high tolerance to ionizing radiation, and a method for using such material to maintain the relative positions of a plurality of dice during high temperature processing and to attach the dice to a substrate by interjecting a solvent to a dried layer of die attach material situated between a die and the substrate.
2. Description of the Prior Art
Thermoplastic materials pass through a specific sequence of property changes as their temperature changes. In particular, as temperature increases, a thermoplastic material will change from a glass to a flexible substance, and then to a liquid. Conversely, as temperature decreases, the property changes will occur in reverse order. The property changes occur at transition temperatures which are specific to the thermoplastic compound.
Thermosetting materials exist initially in the liquid state but, on heating, undergo a reaction to form a solid exhibiting a highly crosslinked molecular matrix. Unlike thermoplastics, the heat reaction of thermosetting materials is not reversible.
Thixotropy is a property of certain colloidal gels, meaning that they are coagulated when at rest but become fluid when agitated or otherwise subjected to stress.
Use of polymeric materials, glue or so-called "die attach" materials to bond an integrated circuit chip or die to a substrate has long been known in the art. Most currently-available die attach materials are epoxy resins, also known as epoxies. Epoxy resins function as thermosetting compounds and are permanently hard once cured. Filler materials can be mixed with an epoxy resin to gain thixotropic attributes. Epoxies can only be slightly softened to allow for die removal and the softening point varies considerably between materials. Epo-Tech resin sold by Epoxy Technology, Inc. of Billerica, Mass. and Able Stick resin sold by the Able Stick Company of Gardenia, Calif. are two examples of epoxy-based die attach materials.
Thermosetting polyimide materials have also been used as die attach materials. Like the epoxies, thermosetting polyimide materials prevent the die from being easily removed from a substrate since the thermosetting bonding material does not soften significantly on heating. Heating to very high temperatures is required to achieve only a slight amount of softening. Moreover, thermosetting polyimides cure by a condensation reaction in which water is produced during the reaction and must be evaporated. This poses a formidable obstacle to bonding multiple chips in a large area or to bonding a single very large scale integrated circuit (VLSI) chip since boiling off the water causes the chips to shift positions, and nonuniformity of bonding has been attributed to such evaporation. Furthermore, not only are temperatures in excess of 250.degree. C. required for some imidization reactions to proceed, but also, thermosetting polyimide resins must be refrigerated in order to be useful. With some thermosetting polyimide resins, B-staging has been observed in less than one week at room temperature. G.E. Silicones of Waterford, N.Y., E.I. du Pont de Nemours and Co. of Wilmington, Del., and M & T Chemicals of Rahway, N.J. are commercial vendors of thermosetting polyimides.
Some thermoplastic materials have been used as die attach materials. Thermoplastic materials are typically available in pellet or film form. The pellets may be solvented by appropriate solutions. Fully-reacted thermoplastic materials which are in solvented form merely need a drying cycle to yield a bond. Drying occurs at relatively low temperatures compared to the high temperatures required for imidization. M & T Chemicals markets a polyimide siloxane material designated 2064 which is used for bonding chips to substrates; however, it is not thixotropic, it softens at a temperature below 100.degree. C. and it readily dissolves in a number of solutions. Therefore, 2064 material has limited utility in chip packaging procedures which require exposing the chips to heat and solvents during processing, since the chips may become weakly attached or be totally lifted during these procedures. Thermoplastic materials which have limited solubility usually must be solvented with n-methylpyrrolidone (NMP). Being extremely hygroscopic, however, NMP imposes a severe limitation on employing a polymer material for die attachment since water molecules in the air are collected by the polymer blend and cause the polymer material to separate out of solution. This separation is usually observable as a white skin that forms over the surface of the polymer blend. The skin tends to keep the polymer blend from wetting the chip and substrate, thereby preventing die attachment.
Many available die attach materials are not tolerant to ionizing radiation, i.e., not radiation hardened, and are, therefore, unacceptable for use in an ionizing radiation environment. Some die attach material fillers may exhibit spalling, resulting in die detachment, when exposed to a dose of high ionizing radiation. Some die attach materials have been found to undergo outgassing in high ionizing radiation environments, which can also result in die detachment.
When joining a plurality of closely spaced dice to a substrate, the normal, large, auto-dispersed dots of glue which are typically used for die attachment tend to ooze out beyond the edges of the chips. The dot masses join together and, by capillary action, draw the chips together. Numerous attempts have been made to correct this so-called "swimming together" problem by using precision displacement dispensers to deposit a precise array of dots to minimize glue spread when die attachment takes place. Commercially available pick-and-place machines, however, tend to require too much time for dispensing the dots in a precise manner. At a dispensing time of 2 seconds per dot, where the diameter of each dot measures approximately 5-10 mils, then for a 4.times.4 inch substrate filled with dice, 10 to 16 minutes can pass before all the dice are attached. Although stamps have been conceived to shorten the glue dot placement time, they have not consistently produced good results.
Epoxy materials which harden with heat have not proven to be satisfactory die attach materials for closely-spaced dice for two reasons: first, the viscosity of epoxy materials drops drastically during cure, thus allowing the dice to swim together and, second, epoxy materials which are time-cured have a tendency to rise to different levels above the substrate surface while curing during the placement of dice on the dots, thus causing variability in chip height and bonding. Pre-imidized polymer materials such as polyetherimides and polyimide siloxanes are available in solution and require the solvent to dry out in order to form a bond. Hygroscopic solvents such as NMP are typically used with these pre-imidized polymer materials and, in humid environments, tend to skin over, as discussed above, on the order of seconds. Such pre-imidized polymer materials would therefore be unsatisfactory for bonding a plurality of dice in a span of 16 minutes. Attempts have been made to obviate the hygroscopicity problem by using diglyme as the solvent, a mixture of dimethyl ethers instead of NMP; however, diglyme is faster drying than NMP and may evaporate from solution before all the dice have been placed on the glue dots. In addition, evaporation of diglyme may make the polyimide blend unstable and thereby limit its wetting ability. Thermosetting polyimide materials, which must be fully imidized to hold dice, undergo imidization at high temperatures and, because the reaction is by condensation, severe out-gassing occurs which can result in die-detachment. Moreover, thermosetting resins cannot be softened after they have been fully cured, making repair of a die difficult if not impossible.